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While Tb8 was considered adequate in predicting most of the development processes, Tb0 was more appropriate for grain filling and field drydown. Other reports citing Tb0 for grain filling in maize include Muchow (1990) and Birch et al. (1998c). Superiority of Tb0 over Tb8 during and post-grain filling was likely because the two processes active during those periods involve a component of physical diffusion of sucrose and grain moisture loss (Brooking, 1990). Unlike other processes such as LA expansion which are enzyme mediated (Purdy and Crane, 1967), water loss does not involve enzymes. Enzymes typically respond to temperature in a Q10 fashion that implies a base temperature and a sharply curvilinear response to temperature over a narrow range. Drying rate is instead influenced more by physical factors such as pericarp permeability and osmotic diffusion (Crane et al., 1959). Diffusion processes on the other hand tend to be proportional to absolute temperatures and to concentration gradients.

Prediction of the timing of occurrence of TI is a basic requirement of most maize process models since this duration is used to predict final leaf number and is key to estimating planting-to-flowering durations. The indirect method for estimating TI (see section 4.4.5) established in this study provides a simple way of determining TI when destructive dissection is not an option. Other researchers have predicted TI to occur at 0.4 x seedling emergence to silking TT duration (Bonhomme et al., 1991). This relationship was evaluated using data from the current study, and was not adopted because it accounted for less variability in TI than a relationship associated with final leaf number (r2=0.50, vs. 0.67). Additionally, leaf number was considered more readily observable, unequivocal, stable, easily measured and less prone to error than TT. A linear relationship established by Hunter et al. (1974) which also depends on final leaf number (TI = 0.44 x final leaf number - 1.95) under-predicted leaf number at TI. This relationship was established using older hybrids and it appears necessary to modify the equation using data from current maize hybrid genetics.

The observed variations in TT durations between emergence and TI among PDs (Table 4.3) possibly suggest that Tb8 may still be too high for this interval, or that photoperiod sensitivity differed among the maturity classes of hybrids. However, even though TT

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from emergence to TI differed among PDs under non-limiting moisture conditions, the variation around the mean was quite small and considered inconsequential. Photoperiodic effects were also not considered to have influenced hybrids differently among PDs due to the narrow range in daylengths observed across PDs. Moreover, modern temperate hybrids are generally not very photosensitive (Bonhomme et al., 1991).

The inverse relationship between TT for the emergence-TI interval and soil temperature immediately prior to TI in the water stressed ENV was consistent with other research (Breuer et al., 1976; Warrington and Kanemasu, 1983c). The relationship was, however, non-existent in ENVs where moisture was not limiting, suggesting that increasing levels of water stress may have triggered this response. Dry conditions appear to have increased soil temperatures by 1-30C for RUK08 vs. RUK07 and it seems likely that the higher temperatures hastened progress to TI by reducing leaf numbers (section 5.5.4.1, Chapter 5), in turn decreasing the duration from emergence to TI. Earlier PD treatments, which were least stressed, had the most number of leaves compared to the last two which received the least amount of rainfall. The reduction in leaf numbers at RUK08 may also reflect a sensing of impending drought that resulted in increased rates of progress towards TI. Decrease in leaf number under drought stress prior to TI has also been reported elsewhere in literature (Jordan, 1983; NeSmith and Ritchie, 1992). The shorter thermal duration from emergence to TI under dry warm conditions, however, depend on the severity of the water stress. For instance, Abrecht and Carberry (1993) reported up to a 4 d delay in time to TI under severe drought stress.

The emergence-flowering duration, which also includes the TI period, is considered here to allow for comparisons with other reports. The average linear decrease in the emergence to flowering duration with delayed planting observed in the Waikato ENVs (-1.1 (0Cd)d-1) falls within the -0.7 to -1.1 (0Cd)d-1 range reported by Nielsen et al. (2002), using Tb10. The response of the emergence-flowering duration to PD in the current study was most likely due to variations in leaf number and phyllochron lengths (see sections 5.4.8 and 5.4.11, Chapter 5). Drought was not considered to have significantly influenced silking duration, since anthesis, which is generally not responsive to drought (Bolaños and Edmeades, 1993b), usually occurred within a day of silking. Increased leaf numbers and/or phyllochron delayed flowering, since

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emergence-silking duration is directly proportional to both variables. Under optimum conditions, TT between emergence and silking can therefore be estimated accurately from total leaf number and phyllochron.

Unlike the Waikato ENVs, MAS08 exhibited a general increase in the duration from emergence to flowering with delayed planting, which concurs with earlier studies on older and possibly more photosensitive hybrids in Canada (Daynard, 1972). This delay in development was largely attributed to a significant increase in phyllochron from 42 to 500Cd between PD1 and PD5 (see section 5.4.11, Chapter 5). Due to the narrow photoperiod range, daylength was not considered to have played a significant role in the present study, though it would have greater influence on leaf number here than in the lower latitudes of the Waikato ENVs. Over all PD treatments, during the last week prior to TI, when photoperiod is known to delay TI in photosensitive maize hybrids (Kiniry et al., 1983b), daylength ranged from 14.2-14.8 h in Waikato and 14.8-15.1 h in Manawatu. Even though higher temperatures just prior to TI for late planting treatments (e.g., 18.7 vs. 14.10C) reduced leaf numbers (see section 5.4.8, Chapter 5), temperature effects were larger on phyllochron than leaf number, resulting in longer emergence-TI durations. Warrington and Kanemasu (1983c) showed that between 15 and 250C, leaf number had a curvilinear response to temperature, whereby the lowest numbers were observed at 180C (see section 5.5.4.1, Chapter 5, for a more detailed discussion). Larger phyllochron values under warmer conditions have also been cited by Tollenaar et al. (1984; 19-290C) and Padilla and Otegui (2005; 12-260C).

Flowering date is a very obvious indicator of overall crop maturity and is expected to be significantly correlated with leaf number and TT duration to TI and maturity. This was the case in the present study. The higher correlations observed between these three traits and anthesis vs. silking were due to the greater sensitivity of silking to environmental stress (Bolaños and Edmeades, 1993b). Anthesis is therefore a more reliable indicator of the flowering growth stage than is silking in maize. Anthesis- silking interval will not be discussed here since the values obtained, even under drought conditions, were not considered large, and correlations between ASI and yield traits were generally non-significant.

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Under non-stressed conditions, grain filling TT durations were generally consistent across PD treatments, especially in Waikato. In Manawatu, a higher latitude ENV where environmental conditions vary significantly between early spring and late autumn, a quadratic response was observed with PD for mid and late hybrids. Similar observations have also been reported under some USA conditions (Nielsen, 2002), further evidence that grain filling duration was not necessarily dependent on temperature alone (Stewart et al., 1998a,b). The latter reported that environmental factors directly affected assimilate availability. For instance, drought or reductions in IPAR and temperature had a significant influence on filling duration. This was evidenced by low KW and KN observed under shorter grain filling durations, which is consistent with assimilate deprivation (Wardlaw, 1972; Stewart et al., 1998a,b).

The higher correlation between grain fill duration and KW vs. KN suggests source limitation during the latter parts of grain filling (Borrás et al., 2004). Drought stress was more pronounced in late hybrids, and these showed an association between duration of grain filling and KW, but not KN. Possible reasons why later hybrids were affected more by stress are discussed in sections 3.5.1 and 3.5.2 (Chapter 3).

In all ENVs the main differences in lifecycle duration between hybrid maturity classes principally reflected variation in grain filling duration. This was evidenced by the higher correlation between the total crop cycle (emergence-PM) and the grain filling durations (r=0.87***) compared to the emergence to silking duration (r=0.56***). While these findings concur with results observed in the temperate areas of Argentina (Capristo et al., 2007), they contradict other reports from studies of diverse maize hybrids grown under varying daylength and temperatures that suggest that variation in the emergence-flowering duration explained the majority of hybrid maturity differences (Derieux and Bonhomme, 1982; Major et al., 1983). This disparity could be due to possible errors associated with Tb assumptions used in these other studies. For instance, while emergence-silking duration for early hybrids was 11% less than for late hybrids, differences in grain filling duration were larger using Tb0 (16%) than Tb8 (13%). Differences may also reflect the fact that hybrids did not differ markedly in duration or photoperiod sensitivity, and that daylengths among PDs and ENVs were fairly similar.

107 4.5.2 Crop growth rate

Crop growth rates in the current study were reported in TT rather than real time, a contrast with most reported literature (e.g., Andrade et al., 1999; Borrás et al., 2007). Use of TT removes artifacts caused by differences in temperature and rate of growth. For instance, at RUK07, the PD1 emergence-flowering duration averaged 108 d, vs. 75 d (PD4) and 67 d (PD5), while the total biomass yields at silking were reasonably similar among PDs (see section 5.4.6, Chapter 5).

The observed increase in CGRES with late planting, particularly at RUK07, was attributed to high RUE, high IPAR (Cirilo and Andrade, 1994a) and near-optimal temperatures for growth between emergence and flowering. Average daily irradiance for the period increased linearly with PD from 19.5 to 21.5 MJ m-2 d-1 between PD1 and 4, before dropping to 21.2 MJ m-2 d-1 for PD5. At the same time mean temperatures increased from 14.4 to 17.70C between PD1 and PD5.

In contrast to CGRES, the general decrease in CGRSS with PD, particularly in the absence of water stress,was attributed to a linear decline in radiation from 20.4 (PD1) to 13.0 MJ m-2 d-1 (PD5). Kiniry and Knievel (1995) showed that KN, which is determined by CGR (Andrade et al., 1999), was linearly related to IPAR. The higher mean temperature regimes between anthesis and silage harvest time for early plantings (18.7 vs. 15.70C) also would have contributed to higher CGRSS through increased RUE as observed by Wilson et al. (1995), who reported low RUE if mean temperatures dropped below 160C.

Even though mean temperature and radiation levels for NGA08 and RUK08 were at least 10C and 1.5 MJ m-2 d-1 greater than RUK07, CGRES rates were 35% less. A similar trend was also observed with CGRSS, where mean temperature and radiation for the two ENVs were at least 1.50C and 2.5 MJ m-2 d-1 higher than RUK07. This was attributed to the effects of water stress. On average, RUK07 received 234 mm rainfall during the emergence-flowering period across the five PD treatments vs. 112 mm for the other two ENVs. The significant decrease in CGRSS, particularly for RUK08, was largely due to lower rainfall during the anthesis to silage maturity period (50 mm) vs. RUK07 (140 mm) negating any direct effect of higher temperature and radiation levels on CGR.

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Reduced CGR under water stress conditions have also been reported by Pandey et al. (2000) and Sangakkara et al. (2004). Similarly, Edmeades (1972) reported that, provided water was not limiting, timing of maximum CGR was governed by trends in temperature and radiation.

The strong correlation between CGRSS and GY compared to CGRES was most likely because sink size is largely determined during and after flowering (Andrade et al., 2000). Additionally, light interception is also more complete during grain filling and thus, higher correlations between CGRSS and yield components are likely as the latter is directly related to IPAR (Kiniry and Knievel, 1995). Post-flowering conditions also have a large bearing on kernel abortion and grain filling (Schussler and Westgate, 1991). Crop growth rate between silking and silage maturity therefore determines GY and its components, as evidenced by the high correlations with GY, HI, KW and KN. Crop growth rate between emergence and silking is however influenced by rate of LA expansion or the speed of canopy closure, directly affecting vegetative biomass. The positive correlations of CGR with KN and green LA are consistent with other findings in literature. For example, Andrade et al. (1999) observed an increase in KN m-2 from 1000 to 3500 when CGR increased from 12 to 25 gm-2 d-1 during flowering.

Averaged across ENVs, under cool, early planting conditions, late hybrids had significantly higher CGRES than early and mid hybrids. This could be due to greater IPAR as a result of quicker canopy closure in late hybrids (see section 5.5.4.2, Chapter 5), or it could reflect higher temperatures experienced by later hybrids immediately prior to flowering. In contrast, the CGRES of early hybrids were equal to or greater than late hybrids under late planting conditions (warm). Similar results have been observed in sweet corn by Garcia et al. (2009) who concluded that since silking occurred earlier in early hybrids, mean temperatures encountered in late plantings were higher compared with later flowering hybrids.